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arXiv:1102.0928 · Physical Review Letters

Continuous Loading of a Conservative Potential Trap from an Atomic Beam

Markus Falkenau, Valentin Volchkov, Jahn Rührig, Axel Griesmaier, Tilman Pfau

Published Apr 22, 2011• 36 citations
arXivHTMLPDF

Abstract

We demonstrate the fast accumulation of 52Cr atoms in a conservative potential from a guided atomic beam. Without laser cooling on a cycling transition, a dissipative step involving optical pumping allows us to load atoms at a rate of 2×10(7) s(-1) in the trap. Within less than 100 ms we reach the collisionally dense regime, from which we produce a Bose-Einstein condensate with subsequent evaporative cooling. This constitutes a new approach to degeneracy where Bose-Einstein condensation can be reached without a closed cycling transition, provided that a slow beam of particles can be produced.

DOI
10.1103/physrevlett.106.163002

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We demonstrate the fast accumulation of 52Cr atoms in a conservative potential from a guided atomic beam. Without laser cooling on a cycling transition, a dissipative step involving optical pumping allows us to load atoms at a rate of 2×10(7) s(-1) in the trap. Within less than 100 ms we reach the collisionally dense regime, from which we produce a Bose-Einstein condensate with subsequent evaporative cooling. This constitutes a new approach to degeneracy where Bose-Einstein condensation can be reached without a closed cycling transition, provided that a slow beam of particles can be produced.
We report on the realization of a sodium Bose–Einstein condensate (BEC) in a combined red-detuned optical dipole trap formed by two beams crossing in a horizontal plane and a third, tightly focused dimple trap (dT) propagating vertically. We produce a BEC in three main steps: loading of the crossed dipole trap from laser-cooled atoms, an intermediate evaporative cooling stage that results in efficient loading of the auxiliary dT, and a final evaporative cooling stage in the dT. Our protocol is implemented in a compact setup and allows us to reach quantum degeneracy even with relatively modest initial atom numbers and available laser power.
We report on the realization of a sodium Bose–Einstein condensate (BEC) in a combined red-detuned optical dipole trap, formed by two beams crossing in a horizontal plane and a third, tightly focused dimple trap propagating vertically. We produce a BEC in three main steps: loading of the crossed dipole trap from laser-cooled atoms, an intermediate evaporative cooling stage which results in efficient loading of the auxiliary dimple trap, and a final evaporative cooling stage in the dimple trap. Our protocol is implemented in a compact setup and allows us to reach quantum degeneracy even with relatively modest initial atom numbers and available laser power. 1.
The atom laser has long been a holy grail within atom physics and with the creation of an atom laser we hope to bring a similar revolution in to the field of atom optics. With the creation of the Bose-Einstein Condensate (BEC) in 1995 the path to an atom laser was initiated. An atom laser is continues source of BEC. In a Bose condensate all the atoms occupy the same quantum state and can be described by the same wave function and phase. With an atom laser the De Broglie wavelength of atoms can be much smaller than the wavelength of light. Due to the ultimate control over the atoms the atom laser is very interesting for atom optics, lithography, metrology, etching and deposition of atoms on a surface. All previous atom lasers have been created from atoms coupled out from an existing Bose-Einstein Condensate. There are different approaches but common to them all is that the duration of the output of the atom laser is limited by the size of the initial BEC and they all have a low flux. This leaves the quest to build a continuous high flux atom laser. An alternative approach to a continuous BEC beam is to channel a continuous ultra cold atomic beam into a magnetic guide and then cool this beam down to degeneracy. Cooling down a continuous beam of atoms faces three large problems: The collision rate has to be large enough for effective rethermalization, since evaporative cooling in 2D is not as effective as in 3D and a large thermal conductivity due to atoms with a high angular momentum causes heating downstream in the guide. We have built a 4 meter magnetic guide that is placed on a downward slope with a magnetic barrier in the end. In the guide we load packets of ultra cold rubidium atoms with a frequency rate large enough for the packets to merge together to form a continuous atomic beam. The atomic beam is supersonic and when the beam reaches the end barrier it will return and collide with itself. The collisions lowers the velocity of the beam into subsonic velocities and a shock wave is created between the two velocity regions. In order to conserve number of particle, momentum and enthalpy the density of the atomic beam passing through the shock wave must increase. We have build such a shock wave in an atomic beam and observed the density increase due to this. As an extra feature having a subsonic beam on a downward slope adds an extra density increase due to gravitational compression. Loading ultra cold atoms into a 3D trap from the dense subsonic beam overcomes the problem with 2D cooling and thermal conductivity. This was done and evaporative cooling was applied creating an unprecedented large number rubidium BEC.
We report on our progress towards realization of a toroidal trap for Bose-Einstein condensates (BECs) of 87Rb using a time-averaged optical dipole potential. To achieve BEC we utilize a 20 W laser operating at 1064 nm to form an optical dipole trap in which we evaporatively cool the gas. Loading of the dipole trap is achieved by overlapping the dipole beams with a standard magnetic optical trap. Evaporative cooling is first performed in a single beam trap, followed by compression and additional confinement with a second orthogonal beam. This prevents stagnation of the evaporative cooling cycle, and allows the gain of 105 in phase space density necessary to achieve quantum degeneracy.
In this thesis, I study optical pumping as a powerful cooling tool for trapped ultra-cold atoms in a highly collisional regime. First application of optical pumping is a continuous loading scheme used to transfer atoms from a guided beam into a hybrid trap. Further, I introduce a Sisyphus cooling scheme based on radio-frequency transitions and optical pumping, operating simultaneously to the accumulation of atoms in the trap. The combined scheme of continuous loading and Sisyphus cooling is demonstrated for a large range of initial conditions of the guided atoms. Thereby, I show that collisional thermalization occurs in a steady-state for almost arbitrary initial conditions, provided that the first dissipative step is able to prevent the atom from leaving the trap during its first passage. On the one hand, this scheme could be applied to a wide range of atomic or molecular beams. On the other hand, phase-space density of 4*10^-4 is reached in a continuous operation mode with chromium atoms. In the second part, I investigate demagnetization cooling based on dipolar relaxation collisions driving the thermalization of the internal (spin) and the external (motional) degrees of freedom. In the case of a gas, one has the advantage that the spin degree of freedom can be cooled very efficiently using optical pumping. It is shown, that demagnetization cooling of a gas is more efficient than evaporation cooling in terms of phase-space density gain versus loss of atoms. This allows reaching a temperature of 6uK at a phase-space density of 0.03. It is observed, that both, continuous Sisyphus cooling and demagnetization cooling are limited by a density dependent loss mechanism. I present circumstantial evidence for excited-state collisions as the dominant limiting process. Finally, I discuss possible extensions to the current experimental procedures, possibly allowing reaching quantum degeneracy by optical means only.
Summary form only given. Recently, we have created a Bose condensate of /sup 87/Rb atoms directly in an optical trap formed by tightly focused laser beams. Following initial loading from a laser cooled gas, evaporative cooling through the BEC transition is achieved by simply lowering the depth of the optical trap. Our success is due in part to a high initial phase space density realized in the loading of our optical dipole trap and in part to the tight confinement of the atoms that permits rapid evaporative cooling to the BEC transition in 2 s.